Preliminary retrospective studies at our institution have indicated an association between viral infections and acute febrile illness, transplant rejection and mortality. However, the following have not been answered satisfactorily: 1) What is the source of the patient's virus? 2) Do viruses cause clinically relevant illness? 3) How do viral infections relate to immune function and episodes of rejection? Our objective is to answer these questions by performing an intensive prospective epidemiologic viral study. The source of virus will be sought by examining serologic studies of donors, transplant staff, and age-matched blood donors. The relationship of viral shedding with clinical illness will be examined by prospective studies with frequent viral culturing and detailed clinical/observations. Specimens will also be examined by immunofluorescence and counterelectrophoresis for evidence of viral antigens. Nonspecific and specific humoral and cell-mediated tests will be done to investigate the relationship of viral infection to immune function, and these studies will be correlated with episodes of rejection. Routine tests will include quantitation of T and B cells, a battery of viral antibody titers, mitogen stimulation using PHA and Con A, and mixed lymphocyte culture. Tests for CMV-specific cell-mediated immunity have been developed and are being used prospectively as indicators or predictors of the presence and severity of CMV infections.